1. Field of the Invention
The present invention relates to power supply devices for high frequency power amplification circuits. In particular, the present invention relates to a power supply device that improves power efficiency of a high frequency power amplification circuit and to a high frequency power amplification device that includes such a power supply device.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2006-514472 describes a configuration in which, in order to enhance the efficiency of a high frequency amplification circuit, a power supply voltage of the high frequency amplification circuit is caused to follow an amplitude change (envelope) of a high frequency signal to be amplified. The frequency of the amplitude change of the high frequency signal here is not a carrier wave frequency (e.g., 1 GHz to 2 GHz in cellular phones) but is the frequency (approximately 10 MHz to 100 MHz) of the amplitude change of the carrier wave. An example of an audio amplification circuit is a class-D amplification circuit (digital amplifier), but in a power supply device for a high frequency power amplification circuit, a frequency band to be handled is a high frequency band, and thus extremely high speed switching is required.
FIG. 1 is a block diagram of a high frequency power amplification circuit 50 described in Japanese Unexamined Patent Application Publication No. 2006-514472. This high frequency power amplification circuit (RF AMPLIFICATION STAGE) 50 illustrated in FIG. 1 includes an envelope detection circuit (ENVELOPE DETECTOR) 54, a supply voltage selection circuit (SUPPLY VOLTAGE SELECTION) 56, a power supply voltage adjustment circuit (SUPPLY VOLTAGE ADJUSTMENT) 58, and a high frequency amplification circuit (RF AMPLIFIER) 52. The envelope detection circuit 54 detects an envelope of a high frequency signal (REIN), and the supply voltage selection circuit 56 selects a voltage source (V1 to V4) so as to follow the envelope of the high frequency signal (REIN). The power supply voltage adjustment circuit 58 adjusts the waveform of the selected voltage and supplies the result to the high frequency amplification circuit as a power supply voltage. Thus, the high frequency amplification circuit 52 amplifies the high frequency signal (REIN) and outputs a high frequency signal (RFOUT).
The high frequency power amplification device of Japanese Unexamined Patent Application Publication No. 2006-514472, however, employs a system in which a plurality of voltage sources are prepared and a voltage source is selected so as to follow the envelope of the high frequency signal. Thus, a complex circuit and extremely complex control are required to repair a voltage difference (waveform step) occurring when switching among the plurality of voltage sources. If this repair of the waveform step is incomplete, distortion in the waveform increases, and an adjacent channel leakage power ratio (ACPR) in a communication system increases. Thus, frequencies outside a stipulated range are negatively affected, leading to a problem that the high frequency power amplification device cannot be applied to a product having strict frequency band allocation.